This invention relates to alunite-type compounds and more particularly it relates to a process for making alunite and a high purity product, e.g., natroalunite resulting therefrom.
The occurrence and geology of the mineral alunite in many parts of the world is widely recognized and well documented. However, such natural deposits contain many impurities such as quartz, kaolin, iron oxides and other minerals which are difficult and expensive to remove. Further, natroalunite, which has sodium in the place of potassium in the chemical formula, is often mixed with alunite and is not found in the pure form in deposits. Ammonium alunite has ammonium in place of potassium. There are no reports of its occurrence in nature.
U.S. Pat. No. 3,436,176 discloses a process for producing high purity alumina from aluminum-bearing acidic, sulfate solutions. According to the patent, the feed solution may be considered as taken from a cyclic backing system of the waste dump of a copper mine, and a sodium or ammonium salt is added to such feed solution. With the salt added, 85% of the aluminum values present in the solution precipitates as sodium or ammonium alunite which contains 35 to 45% aluminum oxide, less than 3% iron, and less than 5% sodium or ammonium ions. Thereafter, the sodium or ammonium alunite is calcined and digested to produce alumina.
However, there is still a great need for a process to produce a high purity alunite, e.g., natroalunite. The present invention provides such a process and produces a high purity alunite.